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GIS Report
1. GIS
A Geographic Information System (GIS) integrates hardware, software, and data for
capturing, managing, analyzing, and displaying all forms of geographically referenced
information. In this project, we get the information of the catchment surface.
2. Data Information
The data given (shapefile): building, catchment, contour, flood area, manhole ground level,
road, sewer network and so on.
3. Terrain pre-processing
The purpose of terrain pre-processing is to perform an initial analysis of the terrain and to
prepare the dataset for further processing. A Digital Elevation Model (DEM) of the study area
is used as input for terrain pre-processing. During the processing, potential problems with
the terrain representation can be identified, thus preventing the DEM errors from
propagating to the later stages of the analysis. A successful pre-processing is an indication
that the underlying DEM does not contain major problems that will prevent further analyses.
The following functions, in order, are involved in terrain pre-processing.
3.1 DEM
First we need to create DEM (Digital Elevation Model), it is used as input to quantify the
characteristics of the land surface. A DEM (figure 1) is a raster representation of a
continuous surface, usually referencing the surface of the earth.
The method: clip (contour + catchment)
clip (raster + catchment)
create TIN
convert TIN to Raster (DEM)
Figure 1. Data map of catchment
3.2 fill sink
Sinks are often errors due to the resolution of the data or rounding of elevations to the
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nearest integer value. It should be filled to ensure proper delineation of basins and streams.
This raster (figure 2) will be used for the next steps.
Figure 2.DEM of catchment
3.3 flow direction
The direction of flow is determined by the direction of steepest descent from each cell. This
function takes our DEM as input and outputs a raster (figure 3) showing the direction of flow
out of each cell.
Figure 3. fill sink map
3.4 Flow accumulation
The Flow Accumulation function calculates accumulated flow as the accumulated weight of
all cells flowing into each downslope cell in the output raster (figure 4). It uses Flow Direction
as input grid.
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Figure 4. flow accumulation map
3.5 Stream definition
The Stream Definition function takes a flow accumulation grid as input and creates a Stream
Grid for a user-defined threshold. The stream grid contains a value of "1" for all the cells in
the input grid that have a value greater than the given threshold. All other cells in the
Stream Grid contain no data.
Figure 5. Stream definition map
3.6 Stream Segmentation
The Stream Segmentation function creates a grid of stream segments that have a unique
identification. Either a segment may be a head segment, or it may be defined as a segment
between two segment junctions. All the cells in a particular segment have the same grid
code that is specific to that segment (figure 6).
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Figure 6. stream segmentation map
4.Vector Processing
4.1 Catchment Grid Delineation
The Catchment Grid Delineation function creates a grid in which each cell carries a value
(grid code) indicating to which catchment the cell belongs. The value corresponds to the
value carried by the stream segment that drains that area, defined in the stream segment
link grid. The Catchment Polygon Processing function takes as input a catchment grid and
converts it into a catchment polygon feature class (figure 7).
Figure 7. catchment grid delineation map
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4.2 Catchment Polygon Processing
The Catchment Polygon Processing function takes as input a Catchment Grid Delineation and
converts it into a catchment polygon feature class (figure 8). The adjacent cells in the grid
that have the same grid code are combined into a single area.
Figure 8. catchment polygon processing map
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5. Conclusion
Considering the result of Stream Segmentation (figure 9), we splice NO.2 to NO.1, and we
get the final subcatchment result (figure 10).
Figure 9. Stream segmentation map
Figure 10. Subcatchment map (Unit: sq m)
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